Work roll

ABSTRACT

Improved work roll, particularly for use in closed gap rolling, has a composite structure comprising a cylindrical outer layer ( 14 ) made of steel, and providing the work surface of the roll, separated from a steel core ( 12 ) by an intermediate layer ( 13 ) of material such as aluminium or copper having a lower Young&#39;s modules than that of the layer ( 14 ). Provided that the dimensions of the outer layer ( 14 ) and intermediate layer ( 13 ) are chosen appropriately, this combination can release a work roll having an affective Young&#39;s modules which is lower than that of steel whilst at the same time retaining the advantages of a steel work surface and enabling higher rolling speeds.

The present invention relates to a work roll, and to a method ofproducing metal foil by rolling metal web, sheets or strips and inparticular by cold rolling of the metal. The invention is particularly,but not exclusively, useful in the production of aluminium or aluminiumalloy foil especially when closed gap rolling is used. The term metal asused in this specification is intended as reference to a metal and/orits metal alloy. For example, the metal may be aluminium or an aluminiumalloy, particularly foil alloys such as AA8006, AA1045 or AA1200.

Methods of cold rolling metal sheets or strips to form metal foil arewell known in the art, and make use of work rolls made of steel having aYoung's modulus of just over 200 GPa. The metal sheet initiallyundergoes open gap rolling until it is thin enough to necessitate closedgap rolling.

With reference to FIG. 1, in closed gap rolling opposing edge regions ofthe circumferential surfaces of the work rolls 1 are in contact beyondthe longitudinal edges of the metal strip 6, so that the thickness ofthe emerging strip is controlled by flattening of the surface of therolls. Here, the longitudinal edges of the metal strip are taken to meanthe edges of the metal strip that lie parallel to the direction oftravel. In open gap rolling the work rolls are not in contact beyond thelongitudinal edges of the metal strip.

International patent application publication number WO99/48627 describesa method of manufacturing metal foil by cold rolling metal plateinitially using work rolls having a Young's modulus between 210 GPa and310 GPa for carrying out open gap rolling, and later using work rollshaving a Young's modulus in excess of 540 GPa for carrying out rollingon at least some of those passes that would have been closed gap ifordinary steel rolls had been used. The final pass or passes are carriedout using work rolls having a Young's modulus between 210 GPa and 310GPa. This document does not mention work rolls having a Young's modulusbelow 210 GPa or use thereof. Indeed, this document suggests that“harder” rolls (by which is meant rolls with a higher Young's Modulus)should be used in near closed gap rolling. Also, it does not address thequestion of how to increase speed of rolling.

DE19702325 describes a method of making rolls and roll sets forvibration dampening. This document teaches the use of a hollow roll or aroll having a longitudinal bore extending along the axis of the rollwhich may be filled with, for example, lead or tungsten to alter thevibration characteristics of the roll. There is no suggestion that thebore or the filling alters the effective Young's Modulus of the roll.

U.S. Pat. No. 2,187,250 describes a method of compensating for rolldeflection. One of the embodiments describes the use of a composite rollcomprising a central shaft separated from the steel outer shell by anintermediate layer of material, such as cast iron, having a relativelylower modulus of elasticity. The central shaft is crown shaped so thatthe intermediate layer is thinner in the middle than at the ends. Inthis way the elastic effect of the intermediate layer is felt more atthe edges of the roll than its centre, thus tending to cancel deflectionforces.

U.S. Pat. No. 3,503,242 describes a composite work roll which isdesigned to overcome the problem of flatness in the rolled product. Theroll is designed to elastically deform locally to correct localoff-flatness in the product. The roll comprises an outer sleeve of hardmaterial separated from a central arbour by a thin layer of low modulusresilient material such as hard rubber or other elastomer, polyurethane,neoprene butadiene-styrene or similar elastomeric materials.

There are many disadvantages associated with conventional techniques forfoil rolling metal sheets. Although the use of work rolls with a highYoung's modulus allows a larger reduction in the gauge of the metal perpass, their disadvantage is that they produce poor flatness of the foil.The use of work rolls with low Young's modulus, whilst avoiding theproblem of poor flatness of the metal, has the disadvantage of limitingreduction through roll flattening on later passes.

In conventional closed gap rolling methods in order to achieve a smallgauge of foil, high loads are used to exert high pressure on the metalthrough the work rolls, but this has the disadvantage that, beyond acertain load only deformation of the work rolls occurs and so no furtherreduction of gauge will result.

A further disadvantage is that presently the speed of the mill is usedto control the exit gauge of the foil. When the rolls are freshly groundand the friction between the rolls and the foil is high, the sheet canbe rolled faster. As the work rolls wear, they become smoother and thefriction decreases. To compensate for the loss of load resulting fromthis, the mill must be slowed down, thus limiting its productivity. Mucheffort is presently spent on maintaining a high friction by the use ofvarious roll grinding techniques. When the work rolls wear smooth theymust be removed from the mill and be re-ground to regain their highfriction properties. In most foil mills, rolls may have to be re-groundafter only a few hours of use.

The inventor has found that reducing the effective Young's modulus of atleast one of the work rolls, although counter-intuitive, addresses theseproblems and in particular allows the speed of the mill to be increased.Accordingly, a first aspect of the present invention provides a workroll for use in the reduction of the thickness of metal sheet, the workroll having an effective Young's modulus in the range 40 GPa to 190 GPa.

According to a second aspect of the present invention there is provideda method for reducing the thickness of metal sheet by cold rollingcomprising the steps of rolling the metal sheet at least once using workrolls at least one of which is a roll having an effective Young'sModulus in the range 40 GPa to 190 GPa.

In a preferred embodiment the effective Young's modulus of the work rollis less than 180 GPa, preferably less than 150 GPa and even morepreferably less than 120 GPa. In this specification, work rolls havingan effective Young's modulus less than 190 GPa will be referred to ascompliant rolls.

Young's modulus is an inherent property of a material: for example theYoung's modulus for steel is about 210 GPa, whilst for aluminium it isabout 70 GPa. Compliance is a property of a structure, being directlyrelated to the displacement (of the structure) in response to a givenload applied thereto. Compliance is dependent therefore not just on theinherent properties of the materials making up the structure—in thiscase the Young's modulus—but also on the structure itself and inparticular, its geometry.

Mathematically, the compliance of a work roll is the deformation of theroll h divided by the roll pressure p, and is defined as follows:

$\begin{matrix}{{Compliance} = \frac{h}{p}} & (1)\end{matrix}$

For the purposes of the present application, compliance can be regardedas directly related to Young's modulus but it should be noted that thecompliance of a work roll will change if its geometry is changed and inparticular if its diameter is changed. Thus, in the present discussion,it is assumed that the geometric properties of the work roll, and inparticular its diameter, are constant. The effect of roll diameter onspeed of rolling will be discussed specifically below.

The reason why compliance is important in the context of this inventionis that it has been found that increasing the compliance of the rollallows the mill speed to be increased. Since Young's modulus is asignificant, although not the only, component of compliance, it will beevident that reducing the Young's modulus which in turn increases thecompliance, all other things being equal, will also allow the mill speedto be increased. The challenge is to provide a work roll which, on theone hand, has such a reduced Young's modulus but, on the other hand, hasa work surface which is hard wearing.

The present invention may replace the need for high friction to achievethin gauge of foil by the use of compliant rolls. This results in theability to roll with smoother work rolls which will allow both fasterrolling and less frequent work roll changes for re-grinding.

In addition, using one or more compliant rolls improves the flatness ofthe sheet. It is believed that off-flatness results from mismatchbetween the thickness profile of the strip entering the roll gap and theroll gap profile. This difference causes an unequal elongation of thestrip from place to place across the strip width and consequentoff-flatness. Local deflections of the rolls across the roll width canaccommodate these differences to some extent. Compliant rolls allowgreater differences to be accommodated without increasing theoff-flatness of the strip exiting the rolling mill. Using compliantrolls, the thickness profile variations of the incoming strip tend toremain as thickness variations rather than being translated into localvariations in length.

In a preferred embodiment, the compliant rolls are positioned, in use,such that opposing edge regions of their circumferential surfaces aretouching the corresponding edge regions of the other work roll or rolls(closed gap rolling). Alternately, the compliant rolls are positioned inuse such that their opposing edge regions are close to touching, but nottouching, the corresponding edge regions of the other work roll or rolls(near closed gap rolling). In both circumstances the compliant rolls maybe used in combination with speed control to achieve the desiredthickness of foil. The compliant rolls allow a higher speed to be usedto obtain a given thickness.

As already mentioned, Young's modulus is a material property and is thusnot easily altered for a given material. In a preferred embodiment ofthe invention there is used a composite work roll that has an effectiveYoung's modulus which is very different from that of solid steel. Such acomposite work roll is preferably fabricated as a core on which ismounted one or more cylindrical layers. In all cases the outermostlayer, forming the work surface of the roll, is made from a relativelyhard material such as steel or chrome to act as a wear resistant surfaceto the roll. Underneath the outermost layer is a layer of compliantmaterial such as aluminium, copper, or magnesium and it is the existenceof this compliant layer that gives the work roll, as a whole, itscompliance. Controlling the thickness of the outer layer (which isassumed to have a high Young's modulus) and the inner layer (which isassumed to have a lower Young's modulus) enables the degree ofcompliance to be adjusted, as will be explained in more detail below.

In a composite roll having just a single cylindrical layer over thecore, then this single layer is the hard outer layer and the core is thecompliant inner layer. Where there are two layers over the core, thenthe core may be made of hard material such as steel, and the compliantinner layer takes the form of an intermediate layer lying between thecore and the outermost layer. Still further layers may be used, ifneeded to achieve particular characteristics.

The outermost layer of the composite roll needs to be reasonably thickotherwise it is difficult to manufacture. In addition it is preferableto provide some excess thickness to allow for regrinding of the roll asit wears. On the other hand, the thicker the outermost layer, the morethe effect of the compliant inner layer will be masked, and a compromisetherefore has to be reached. The absolute minimum radial thickness isprobably about 4 mm with about 5 mm as the preferred minimum thickness.However, outer layers which are even thinner than this may be used, andhave the advantage or providing the maximum compliance. For example,metal or carbide outer layers only 20 microns thick can be applied byflame or plasma spray deposition. When worn, these layers may berepaired by re-spraying rather than regrinding. The maximum radialthickness to avoid excessive masking of the effect of the inner layer is20 mm. However, the maximum thickness is preferably about 15 mm and evenmore preferably about 8 mm.

In embodiments where the inner layer takes the form of an intermediatelayer positioned between a core and the outermost layer, theintermediate layer is preferably between 20 to 40 mm thick. However somebenefit is obtained with quite a thin intermediate layer, say down to 5mm, but the preferred minimum thickness is 10 mm. In any event, it ispreferred that the outermost layer is thinner, in the radial direction,than the intermediate layer.

Preferably the Young's Modulus of the material of the inner layer liesin the range of about 40 GPa to about 150 GPa, this covering magnesiumat the lower end (about 44 GPa) and copper at the upper end (about 120GPa). The preferred range however is 40 GPa to 120 GPa, with aluminium(about 70 GPa) as the preferred material. Materials with very lowYoung's Modulus, for example flexible plastics material or rubber, arenot suitable since these will not have sufficient strength to transmitthe very considerable drive forces which are involved in the rolling ofmetals. Moreover the outer shell of the roll will be able to move toomuch relative to the core. In addition, such materials will tend to actsimply in the manner of a flexible coupling, and will not provide thecomposite effect required. Rather, what is needed is a structure inwhich the outermost hard layer and the inner compliant layer acttogether to provide the required effective Young's Modulus.

A work roll having a composite structure such as discussed above willhave a Young's modulus which is the resultant of the Young's modulus ofthe component materials making up the roll. Such a work roll is said tohave an effective Young's modulus such that the elastic response of thecomposite roll to an applied load is the same as that of a solid roll ofthe same external dimensions made of a material having a Young's modulusequal to the effective Young's modulus.

A way of calculating/measuring the effective Young's modulus of such acomposite roll will be described below with reference to FIG. 2 whichshows part of the circumference of a work roll 1.

If the elastic surface deflection of a composite roll 1 under a givenpressure is h, the effective Young's Modulus of that roll is equal tothe Young's Modulus of a material which when made into a solid roll ofthe same diameter, is deflected by the same amount, h, under the samepressure.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of closed gap rolling of metal sheettaken perpendicular to the direction of travel of the metal sheet;

FIG. 2 is a diagram illustrating how an effective Young's modulus may becalculated;

FIG. 3 is a cross-sectional view of a first embodiment of a compliantroll having a composite structure in accordance with the presentinvention;

FIG. 4 is a graph of relative deformation against layer thickness andeffective Young's Modulus for the embodiment of FIG. 3;

FIG. 5 is a cross-sectional view of an alternative embodiment of acompliant roll having a composite structure in accordance with thepresent invention;

FIGS. 6A–C are graphs of relative deformation against layer thicknessand effective Young's Modulus for the embodiment of FIG. 5;

FIG. 7 is a graph of throughput speed against layer thickness for theembodiment of FIG. 5;

FIG. 8 is a graph of throughput normalised speed against normalisedeffective Young's modulus; and

FIG. 9 is a cross-sectional view of a compliant roll taken perpendicularto its axis of rotation showing the distribution of stresses in the rollof the present invention.

Referring to FIG. 3, there is shown a compliant roll 1 having acomposite structure. The roll 1 has a work surface which takes the formof an outer layer 10 made from a material with a high Young's Modulus,for example steel, whilst having an interior 11 made from a materialwith a lower Young's modulus, for example aluminium or copper, therebyallowing the work roll to have a work surface which is hard wearingwhilst having an effective Young's modulus that is less than that ofsteel.

For example, the composite roll of FIG. 3 may consist of an aluminiumcore 11 having a Young's modulus of about 70 GPa with an outer steellayer 10 having a Young's modulus of about 210 GPa. The purpose here isto create a work roll with a work surface that is hard wearing and whichwill not stick to the sheet being rolled, but which has an effectiveYoung's modulus and compliance nearer to that of the underlyingaluminium. The aluminium core 11 is cylindrical in shape and has aconstant cross section along the working length of the roll. The outersteel layer 10 is of a hollow cylindrical shape and is joined to thecore 11 by any suitable means, such as shrink fitting or adhesive, orthe core may be cast in situ into the interior of the cylindrical layer10.

FIG. 4 shows how the effective Young's modulus of the composite workroll can be varied by changing the thickness of the outer steel layer10. The vertical axis shows relative deformation, which is in arbitraryunits of length, being the response of the elastic system to theapplication of an arbitrary loading. The only requirement is that thedeformation is small compared with the roll diameter. The leftmostdotted curve represents the total deformation of the roll when a forceis applied across the roll diameter, as during the rolling operation.Some of this deformation is due to bulk deformation of the roll—itbecomes slightly elliptical—and the remainder is due to localdeformation at the roll surface around the point of contact. It is thislocal deformation that is most relevant to closed gap rolling and theleftmost solid curve thus shows the local deformation only. It will benoted that the local deformation becomes insensitive to layer thicknessat much smaller thicknesses than the total deformation; this is to beexpected since the surface effects penetrate only a small distance intothe roll. It will also be noted that, when the outer layer is relativelythick (more than 20 mm), the deformation is similar to that of solidsteel whereas when the layer is relatively thin (for example 1 mm) it isvery similar to that of aluminium.

Therefore a 1 mm thick sleeve of steel shrunk on to an aluminium basewould produce a roll with a compliance only slightly lower than that ofpure aluminium and so the roll would have a Young's modulus of 43% thatof steel. Using such a composite structure for the work rolls would thusallow foil to be rolled about 50% faster than the conventional speedsemployed with steel work rolls.

The two rightmost curves indicate the effective Young's modulus forvarying thicknesses of layer 10. As before, there are two curves: asolid curve for total deformation and a dotted curve for localdeformation. Both curves illustrate how the deformation of the surfacevaries with Young's modulus from a value of about 70 GPa (correspondingto solid aluminium), to 200 GPa (corresponding to just less than solidsteel). This deformation is related to the gauge which would be rolledin closed gap rolling which, with solid steel rolls can vary from about0.050 to about 0.006 mm. With compliant rolls, closed gap rolling couldbegin at a thicker gauge. In practice, the mill speed would be increasedto bring about closed gap rolling at about the same gauge as it wouldoccur with solid steel rolls.

The effective Young's modulus of the composite work roll shown in FIG. 3may be obtained by first reading up from a particular value of layerthickness to the local deformation/layer thickness curve and thenreading across horizontally to the local deformation/Young's moduluscurve to obtain the value of Young's modulus. For example, if thethickness of layer 10 is 10 mm, this can be read from the curve of localdeformation/layer thickness to a value of approximately 0.55 which isthen read across horizontally to the local deformation/Young's moduluscurve to give a value of effective Young's modulus of approximately 152GPa.

A similar structure for a compliant roll is shown in FIG. 5. In thiscase the roll consists of a core 12 of steel with an intermediate layer13 made of a lower Young's Modulus material such as aluminium, and anouter layer 14 made of a higher Young's modulus material such as steelor chrome. The intermediate layer 13 has a uniform cross section alongthe length of the roll—i.e. its radial thickness is the same at allpoints along the roll. This structure can be produced using an old, wornsteel roll to which the aluminium intermediate layer 13 is appliedeither mechanically in the form of a sleeve or for example using flamespraying. Finally, the outer layer 14 is applied over the top of theintermediate layer. The outer layer 14 may be applied for exampleelectrolytically or by means of flame or plasma spraying.

The graphs of FIG. 6 correspond to those of FIG. 4, but for theembodiment of FIG. 5. Each of the graphs A, B and C in FIG. 6 plot therelative deformation against the radial thickness of an outer layer 14of steel for a different radial thickness of the intermediate layer 13:

FIG. 6A—intermediate layer thickness=10 mm

FIG. 6B—intermediate layer thickness=20 mm

FIG. 6C—intermediate layer thickness=30 mm

FIG. 6A shows that when the steel outer layer 14 is 10 mm thick, theeffective Young's modulus is about 180 GPa, which is quite close to thesolid steel value of 210 GPa. The corresponding values for FIGS. 6B and6C are 163 GPa and 157 GPa respectively.

FIG. 7 illustrates the effect of different thicknesses of outer layer 14on the speed of closed gap rolling. In particular, the graph shows theenhancement in speed which is obtainable over the use of solid steelrolls. The graph relates to the embodiment of FIG. 5, and separatelycharts the speeds for different radial thicknesses of the intermediatelayer 13. The lower curve is for an intermediate layer of 10 mmthickness aluminium, the middle curve is 20 mm thick aluminium and theupper curve is 30 mm thick aluminium.

FIG. 8 illustrates more directly the effect of different effectiveYoung's moduli on the speed of closed gap rolling. The horizontal axisof the graph is a dimensionless quantity representing the measuredeffective Young's modulus compared with that of solid steel. The graphis valid for all rolls within the normal expected size range and or allrepresentative loads. The three curves coincide for mill loads of 0.5 to0.7 kN/mm, these values being typical of those found in closed gap foilrolling. Mill load is expressed as load in kN per unit width of theroll.

As already mentioned, it is advantageous to use as thick a steel outerlayer as possible because this layer is subject to frequent re-grindsduring service. For this reason it is also advantageous to use arelatively thick under-lying aluminium layer in order to achieve aneffective Young's Modulus which gives significant speed increase.However, increasing the layer thickness above 30 mm has only arelatively small effect as can be seen by comparing FIGS. 6B and 6C whenonly a small reduction in the effective Young's Modulus was achieved fora layer thickness change from 20 to 30 mm.

Furthermore, increasing the aluminium layer thickness too much can causeanother problem. The higher thermal expansion coefficient of aluminiumcauses an increase in the thermal expansion of the roll which can giverise to an increase in thermal camber (different in expansion betweenstrip centre and strip edge)—which can give rise to strip flatnessproblems. For these reasons a suitable combination of intermediate andouter layer thicknesses is 30 mm for the intermediate (aluminium) layerand between 5 and 8 mm for the outer (steel) layer.

Other composite structures having an effective Young's modulus less thanthat of steel can be envisaged that would be in accordance with thepresent invention and an example is given below.

Using a set of compliant rolls as described above, a given rollflattening is obtained using a lower load by using a material or rollstructure with a low effective Young's Modulus. Counter to conventionalthinking, this allows the mill to be operated at a higher speed forgiven gauge and friction conditions.

The amount of roll flattening is also affected by the local pressurebetween the sheet and work rolls. This pressure decreases withincreasing speed. Thus, increasing speed decreases roll flattening anddecreases the rolled thickness. Because of this effect, speed isnormally used to control the gauge of the sheet. Going faster makes thesheet thinner, going slower makes the sheet thicker. Making the workrolls more compliant, as in the present invention, allows the same gaugeof foil to be rolled with the mill running faster. This has greatproductivity benefits.

The effect of changing the Young's modulus of the work roll isillustrated below in the table of results from a theoretical modelaccording to the present invention.

Speed % Speed change for 50% change decrease in Young's Parameter BaseValue [m/min] Modulus WR elastic modulus 2.04 (steel) 607 38% [×10¹¹ Pa]

It can be seen from the table above that a 50% change in Young's moduluscan yield a 38% change in rolling speed. Thus, if the Young's modulus ofthe work rolls is halved to 100 GPa, the rolling speeds can be increasedby 38% for the same gauge and other conditions.

The distribution of stresses in such a composite work roll is shown inFIG. 9 which illustrates a portion of the outer shell 10/14 of the roll.It can be seen that the stresses from contact with the metal sheet arevery intense in the locality 15 of the contact, but are also present inthe substrate. The characteristics of the substrate will thereforeinfluence the effective Young's modulus of a composite roll.

Although the embodiments described above by way of example refer toclosed gap rolling, the method of the present invention may also beapplied advantageously to near closed gap rolling and to open gap coldrolling. Near closed gap rolling refers to when the work rolls arepositioned in use such that the opposing edges of their circumferentialsurfaces are very close but not touching.

Compliant rolls may also be used when metal sheet is pack rolled,thereby minimising the number of passes needed for producing foil of arequired gauge. Pack rolling is the process where a sandwich of two ormore layers of metal sheet is fed into the roll gap between the workrolls. On separating the two or more sheets, the sides in contact withthe rolls have a shiny surface and the inner surfaces are matt.

The presence of front and back tension in the plane of the metal sheetcan also materially affect the rolling load. Back tension is about twiceas effective in reducing the rolling load as front tension. It istherefore a preferred feature of the method of using the compliant rollsto have a back tension applied to the metal sheet during the rollingprocess.

It is further preferred to use a mill with four high stands with workrolls having small diameters of typically between 200 to 450 mm and backup rolls having diameters of typically between 800 and 1000 mm. Using assmall diameter work rolls helps to reduce the rolling load throughreduced arc if contact between the strip and the work rolls. However,the present invention is not restricted to the use of four high millsand other types of mills may be used advantageously.

It was mentioned above that roll diameter, being a component of theoverall compliance of the roll, will also have an effect on the speed ofrolling. In fact, the effect of roll diameter on the speed of rolling inclosed gap mode is very similar to that of effective Young's modulus.Thus a 10% decease in effective Young's modulus would give the sameeffect as a 10% increase in roll diameter. However, since mills aredesigned to run with only a very small range of roll diameters, it ismuch more convenient to use the effective Young's modulus than the rolldiameter as a means of speed increase.

1. A work roll for use in the reduction of the thickness of metal sheet,the work roll having an effective Young's Modulus in the range 40 GPa to190 GPa, wherein the roll has a composite structure comprising anoutermost cylindrical layer and at least one cylindrical inner layer,joined to the outermost layer, and wherein the Young's modulus of thematerial of the inner layer is lower than that of the outermost layer,and wherein the inner layer comprises a cylindrical core of the roll andan intermediate layer positioned coaxially between the outermost layerand the cylindrical core, and wherein the intermediate layer has aconstant radial thickness along the length of the roll.
 2. A work rollas claimed in claim 1 wherein the intermediate layer has a radialthickness in the ranges 5 mm to 40 mm.
 3. A work roll as claimed inclaim 2 wherein the intermediate layer has a radial thickness in therange 20 mm to 40 mm.